In mills for piercing round billets for the manufacture of seamless steel tubing or the like, the billet is advanced along the pass line of the mill between two sizing rolls on axes inclined at small angles with the pass line of the mill. The roll surfaces are contoured so that the space between the rolls converges toward the delivery side of the mill to a minimum called the gorge and then diverges to form the outlet pass. The shafts of the rolls are mounted in bearings that can be moved toward and away from the pass line to accommodate the particular size of billet being processed. Between the guide rolls in the pass outlet, a projectile-shaped piercing plug is held in position on the end of a water cooled mandrel support bar. The point of the piercing plug extends just beyond the gorge toward the feed side of the rolls.
A solid round billet is heated and transferred to a trough on the feed side of the mill. The trough positions the billet at the pass line. The heated billet is pushed forward into the space between the rolls. Because of the angle of the sizing rolls axes relative to the pass line, the billet is rotated and axially advanced through the mill over a piercing plug.
Much the same equipment is used to elongate a tube in an elongating mill. The projectile-shaped piercing plug is replaced with a water cooled mandrel support bar. The sizing rolls are shaped somewhat differently. The heated tube is transferred to the trough on the feed side of the mill and advanced between the sizing rolls and over the working bar.
In one type of seamless tube mill known as a disk-type mill, the billet is held on the correct pass line as it moves through the mill by two disk-type guide rolls having concave toroidal edges that embrace portions of the surface of the billet on opposite sides of the billet.
Since the diameter of the concave toroidal surfaces on the rim of the disk-type guide rolls (hereafter "disk rolls") must be substantially the same as the billet or tube being guided through the mill, the disk rolls must be easily removed and replaced when the tube diameter is changed or when the disk rolls become worn. In the past, the removal and replacement has been very time consuming. The drives to the axles of the disk rolls were disconnected to permit movement of the rolls to a position where they could be removed and replaced. The disconnection, reconnection and alignment of the drives was time-consuming work.
Several schemes have been proposed to avoid disconnecting the drives. In one, the disk rolls are mounted to rotate in a vertical plane while guiding the billet or seamless tube. The disk is held by a crane hook and pushed off the arbor of its axle shaft and lifted up out of the mill housing. In another arrangement, the disks are horizontal. At the time of exchanging a disk roll, the disk roll is released into an arm and the arm pivots out of the mill housing. These designs have drawbacks even though it is not necessary to disconnect the power train to change disk rolls.